Due to advances in industry and requirements of higher intergration, thin film devices are on the rise. As a technology advances, there is a greater need for more uniform thin films having a few or a few tens of nm thickness in highly integrated circuits, optical electron devices and display devices. A thin film device is recognized as a very important device in various applications such as in an insulation layer and an active layer of a semiconductor device, a transparent electrode of a LCD device, and a light emitting layer and a protection layer of an electroluminescence (EL) device.
Here, the thickness of a thin film is a very important control factor which affects fabrication yield and reliability. A need for a real time measurement is increased for a process control in fabrication of such semiconductors and optical devices.
In addition, various thicknesses of the thin film is needed from a few Å to a few hundreds of nm or from a few tens of μm to a few hundreds of μm.
A conventional method (Korean patent laid-open No. 4411) is directed to a mechanical method which is implemented using a probe for measuring a thickness of a thin film. In the above method, there are provided an evaporation source for evaporating a thin film material and a substrate provided in a vacuum chamber having a probe for generating a plasma of a material evaporated. The probe and the substrate are arranged at a certain interval in the vacuum chamber. A first parameter among a plurality of operation parameters which include a voltage applied between the probed and a reference electric potential level and a current flowing through the probe and the plasma is fixed as a fixed value during a certain time interval. A second parameter value among the probe operation variables which are changed in proportion to a deposition rate of the material evaporated on a substrate is measured for thereby measuring the thickness of the thin film.
However, the above mechanical method which uses a probe for measuring the thickness of a thin film may affect the thin film which will be measured, and in the above method, the accuracy of the measured value may be poor if the thin film has a soft surface.
In order to overcome the above problems, the present invention provides a method which is based on an optical process and does not affect the thin film when measuring its thickness.
As an optical method for determining a thickness of a thin film, there are a method implemented using an ellipsometer which is well adapted to an analysis of a thin film including a measurement of a thickness and thickness distribution of an oxide film in a semiconductor industry or by measuring a reflectance, a transmittance, etc. in such a manner that a correct information concerning a physical property and structure of a sample is obtained through a measurement and analysis of a polarized state and variation of light, and a method which is implemented using an interferometer. In the above methods, it is possible to easily measure a thickness of a thin film from a few Å to a few, μm without affecting the sample to be measured.
FIG. 1 is a view illustrating a thin film measurement apparatus using a conventional ellipsometer according to the prior art. The ellipsometer is composed of a polarizer module 101 and an analyzer 102. The light from a light source is polarized by the polarizer module 101 and is directed towards thin film substrate 103. The light reflected by the substrate 103 is collected by the analyzer module 102, and a polarized state of the reflection light is detected by the detector. The detector compares the polarized state of the incident light with a polarized state of the reflection light and obtains an optical coefficient (refraction index or attenuation coefficient) of the film's thickness. However, with the film thickness measuring apparatus of this type, it is impossible to measure the thin film thickness when there are vertical vibrations, inclination and shift in the substrate 103. In addition, in the fabrication line of the LCD apparatus which uses a large thin film glass substrate having a thickness of about 0.5-1.1 mm and a size of a few hundreds square of mm, there are a warping (partial inclination), Vibration, etc. in the substrate. In the case that the thickness of the thin film is below a few μm, when a thickness of a few tens of μm of the thick film is analyzed based on a quantitative light intensity measuring method such as a thin film analysis by an ellipsometer method or a reflection index measurement, the thickness is measured based on a vibration period which occurs in a reflection index or a transmissivity due to an interference effect by a thin film. In particular, in the case that the thickness is a few tens of μm or a few hundreds of μm, there are very difficult requirements in a wavelength resolution of a spectroscope, a ling source or a coherency of a thin film in order to implement a desired interference effect by a thick film. Therefore, it is impossible to determine the thickness of a thick film based on the conventional optical method. In the case that a wavelength resolution or coherency of a spectroscope is poor, a method in which a vibration period (number of vibrations) based on an interference is obtained, and a thickness of a thin film is determined, is effectively used. However, in the case of the thick film, the period has a large value, and the time needed for computing a thin film thickness is increased. In addition, it is needed to increase a vibration period of a spectrum in order to meet a customer's demand that the thickness of a thick film is easily and fast measured. If a wavelength resolution is low, it is impossible to fast check the period of the vibration.
In order to implement the period of the vibration of the spectrum fast, the fast Fourier transformation (FFT) is used. However, in the above FFT, the interval of the peaks is slightly decreased since a refraction index of a thin film is not adapted, and since there are different diffraction indexes based on the wavelength, the values of the thickness obtained based on the value of the refraction index which determines the position of the Fourier peak are changed for thereby decreasing the size of the peak and increasing the width.
Namely, since the periods of the vibrations are not constant, there is a problem in the accuracy of the film thickness.